A current technical trend for DC-DC converters includes downsizing, high efficiency and large capacity. In order to achieve downsizing, a system circuit of a board level is integrated into an integrated circuit (IC) level. Moreover, various techniques such as a current sensing technique have been proposed to implement the high efficiency.
On the contrary, a size of a power transistor (POWER TR) tends to increase for the large capacity. Yet, a size for power capacity is ongoing to decrease. Accordingly, if a large-scale current is detected by a current sensing method, such a disadvantage as downgraded efficiency, speed reduction, increasing errors and the like is considerable caused to degrade performance of a overall system.
FIG. 1 is a diagram of a current sensing circuit for a sensing method in which a buck converter senses a current of a high-side switch.
As illustrated in FIG. 1, if a current flows out of a power TR 12 as a power switch, a negative feedback, which includes a sensing transistor (sense TR) 32, an OP-AMP 34 and a common gate TR 36, senses the current at a predetermined ratio.
Although this method causes no problem if on-resistance (Rdson) is equal to or greater than 1 ohm, it may cause or accelerate a variety of problems. For instance, if a ratio of Iswitch to Isense is raised by decreasing the on-resistance, current sensing error increases. Secondly, if a ratio of Iswitch to Isense is lowered by decreasing the on-resistance (Rdson), the Isense increases. Hence, sensing efficiency is abruptly lowered. Moreover, since the negative feedback is configured using the OP-AMP 34, limitation is put on a bandwidth. Still, an offset is generated by a gain of the OP-AMP 34, if a temperature varies, a signal waveform is distorted by a temperature coefficient difference of the common gate TR 36, the power TR 12 and the sense TR 32. Yet and still, distortion of a sensed current value is generated according to an input voltage range of the OP-AMP 34. Even still, it is unable to sense a fine current value (e.g., 100 mA˜3 A) due to the limitation of the output voltage range of the OP-AMP 34.
Meanwhile, in order to raise efficiency of a DC-DC converter and implement a precise control thereof, various techniques for sensing the current flowing through the power TR 12 of the power switch are ongoing to be developed. These current sensing techniques are mainly categorized into a current sensing method by connecting resistors in series and a current sensing method via the sense transistor 32 using the same device as the power TR 12. However, as the capacity of the DC-DC converter increases, the above-mentioned problems will either be generated or become worsened.